The invention relates to a vehicle brake system. In particular, the invention relates to a coordination of the functions of hydraulic brake boosting and electronic brake force distribution.
Vehicles that have brake systems actuated hydraulically or electrohydraulically conventionally comprise a plurality of mutually independent brake circuits. The use of a plurality of mutually independent brake circuits serves the purpose of redundancy, so that it is still possible to brake the vehicle even if one of the brake circuits should be unable to function. Furthermore, a braking response of the vehicle may be influenced in that during a braking operation the brake circuits are actuated differently. Especially in the case of heavy cars and lorries it is customary to provide one brake circuit for a front axle and another brake circuit for a rear axle. Such a brake circuit split is also known as a “black-and-white split” or “front/rear split brake circuit”.
In many motor vehicles actuation of the brake system is made easier for the driver by means of brake force boosting. In this case, the force generated by the driver by means of an actuation of the brake pedal is transmitted to a master cylinder and additionally increased by a specific factor by means of a brake booster. This assistance makes it easier for the driver to achieve a high brake pressure and hence a high deceleration effect, while as a result of the direct introduction of the hydraulic pressure generated by the driver into the brake circuit a braking of the motor vehicle itself remains possible even in the event of failure of the brake boosting.
The additional force needed to boost the brake force summoned up by the driver is drawn for example by a vacuum brake booster from a vacuum that is conventionally generated by a drive motor of the motor vehicle in a vacuum accumulator. In this case, there are a number of situations, in which such a brake booster is not, or is insufficiently available to assist a braking operation to a required extent. For example, the vacuum accumulator gradually empties while the drive motor is not running. After starting of the drive motor the vacuum accumulator therefore first has to be re-evacuated, which takes a specific time, during which the capacity of the brake booster is not fully deployable. In another example, the motor vehicle is exposed to a low external air pressure, for example at high altitude, so that a boosting effect of the vacuum brake booster because of the slight difference between the vacuum and the air pressure acting from outside may likewise be lower than that requested by the driver.
In order to bridge such situations of insufficient brake boosting, in the background art it is known to provide a hydraulic pump that is designed, where necessary, to generate an assisting hydraulic pressure in a brake circuit (hydraulic brake boost, HBB). A correspondingly powerful design of the hydraulic pump additionally allows the vacuum brake booster to be of smaller dimensions, thereby leading to reduced costs.
If the driver of a vehicle requests a very high brake pressure, then because of the dynamic axle-load distribution of the vehicle during braking a braking effect that may be generated via the front axle is greater than that of a rear axle. Wheels of the rear axle therefore tend to have a higher wheel slip than wheels of the front axle. An undesirable reduction of the directional stability of the vehicle that results from an excessive wheel slip at wheels of the rear axle may be counteracted by an electronic brake force distribution (EBD) becoming active at the rear axle. The EBD reduces the brake pressure of the rear axle brakes by means of an actuation of suitable valves in the rear-axle brake circuit, with the result that the wheel slip at the rear axle is limited.
In certain operating situations the hydraulic brake boosting attempts to increase a brake pressure in a rear axle brake, while the electronic brake force distribution however does not allow the brake pressure generated by the hydraulic pump to reach the brakes. As a result, the HBB system and the EBD system work against one another in these situations.
The underlying problem of the invention is therefore to indicate a method of operating a brake system that avoids the previously described drawbacks.